The grafted together design of the abominations with a view toward functionality (third arm and bulk) might lead you to believe that unnecessary bits for their role might have been left out. Their apparent lack of genitalia would seem to bear this out.

Too, they are described as simpleminded.

The juxtaposition of these two apparently dichotomous gender observations can be resolved by assuming that they are truly neuter, and merely responding to any display of positive attention; the simplemindedness would bear this out.

Patch 3.3.0 (08-Dec-2009): Most Undercity Guardians are being replaced by Kor'kron Overseers.The reason before the Wrathgate was to fill in for the abominations whom Sylvanas send to Northrend to help the Hand of Vengeance, the reason after of course is to keep a eye on the apothecaries.[1]